2200degC Fuel Centerline Thermocouples for the Loft Program

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Abstract
The technology, as well as commercial suppliers, have been developed for high temperature thermocouples for the Loss-of-Fluid-Test (LOFT)* program. Two types of thermocouples were developed and tested. Model B units contained a 1.6 mm (1/16-inch) O. D., 61 cm (24-inch) long Mo/Re sheath probe and were capable of temperature measurement to 1550°C. Model A units contained 1.6 mm O. D., 104 cm long W/Re-augmented sheath probe and were capable of temperature measurement to 2200°C. Both thermocouples were insulated with sintered HfO2 insulators and contained W/5Re-W/26Re thermoelement wires. Both probe sheaths were terminated with an Al2O3 ceramic-to-metal seal capable of maintaining hermeticity to 10-9 cc He/s during temperature transients to 360°C/ min. at 7.2 MPa (2500 psi) continuous and pressure transients of 2.4 MPa (350 psi)/s at 450°C continuous. The ceramic seal and thermoelement wires were connected by means of a transition assembly to a 7.6 m (25 ft.) length of stainless steel sheath MgO insulated cable for signal transmission to the ex-vessel reactor measurement instrumentation. The thermocouples operated for 1,000 hours at their maximum operating temperatures of 2200°C (model A) and 1550°C (model B) with less than 2% drift in the EMF output. The thermocouples also survived thermal transients of up to 145°C/s with no measureable degredation in measurement performance. The time response of the thermocouples was measured to be less than 2-1/2 seconds at 2000°C and less than 5 seconds at 1500°C. The wire-to-sheath insulation resistance after temperature testing remained >1012 ohms.

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